In general, inkjet printing machines or printers include at least one printhead unit which ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink can be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto media or an imaging member. Such printheads can generate temperatures of approximately 110 to 120 degrees Celsius. Once the ejected ink is on media, the ink droplets solidify. The printhead unit ejects ink from a plurality of inkjet nozzles, also known as ejectors.
The media used in direct printers can be in web form. In a web printer, a continuous supply of media, typically provided in a media roller, is entrained onto rollers which are driven by motors. The motors and rollers pull the web from the supply roller through the printer to a take-up roller. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path.
Some continuous feed inkjet printers form printed images on only a first side of the continuous web, a process referred to as a simplex printing operation. Simplex continuous feed inkjet printers have printhead assemblies with printheads which are configured to eject ink across a printing zone on the continuous web which is less than the width of the web. The printing zone is typically centered on the web with appropriate margins on each side of the printing zone. During a simplex printing operation, the continuous web makes only one pass through the printer. Specifically, a rewinder pulls the continuous web through the printer along the web path only once during a simplex printing operation.
Some continuous feed inkjet printers are configured to form printed images on a first and a second side of the continuous web, which is known as a duplex printing operation. In a duplex printing operation, the continuous web makes two passes through the printer, and is referred to as a half-width dual-pass duplex printing operation. In particular, the continuous web is routed from a web supply through the printer to receive ink on the first side. After the continuous web exits the printer, the continuous web is inverted by an inverting system and is then routed again through the printer to receive ink on the second side.
Web transport systems are used in a variety of applications to transport a web from one location to another. In printing applications, a printing assembly including one or multiple print heads positioned near the web prints patterns onto the web. As the ink is ejected on to the web, the web must remain flat and a predictable distance away from the printing assembly. Web unevenness or variations in distance from the printing assembly can result in poor printing quality. The flatness of the web under a printhead includes two sources of errors. As the web moves under the printhead, the out of plane vibration excited by roller eccentricity and bending stiffness of the web around a roller causes the drop flight time to change which provides process direction drop arrival errors. The second error results from web distortion due to troughing wrinkles of the web in the span between two rollers related to web thickness, width, Rh, and tension. A “trough” wrinkle is a wrinkle with a shallow “U”. As the web tension becomes higher, the troughing amplitudes become higher as well. For a typical 20 inch wide web having a thickness of 4 mil, a tension at 3 pli, and a span of 13.1 inch, the wavelength of the troughs are approximately 2.18 inches in length at a height of 0.027 inch. The head spacing from the paper is therefore approximately 1 mm paper in an aqueous ink system and 0.5 mm in a phase change ink printing system. Therefore; both the amplitude of the out of plane vibration and troughing at high tensions can alter the flight time error and possibly allow the paper to touch the printhead surface.
To ensure web flatness, one solution often implemented in the prior art is to stretch the web between two rollers wherein printheads deposit ink on the moving web. The typical arrangement is to print between two rollers. In another embodiment, printing assemblies are located between the rollers and print upon the web which is supported by a vacuum platen which pulls web to the platen to provide a relatively stable printing surface. Vacuum is also referred to as negative pressure herein.
In still another embodiment, the printing assemblies are located in close proximity to the surface of the roller. By printing on the web at a web supported location provided by the roller surface, the web remains relatively stable to provide a stable platform for the deposition of ink. Placing the printhead directly over the tangent of the roller reduces the free span out of plane vibrations and troughing error as implemented on a known phase change ink printer.
In the above embodiments, however, fluttering and troughing of the web affects the stability of the web and thereby introduces printing errors. In the embodiment where the web is supported only by tension where the printing assemblies print, more rollers can be added to the web path to prevent this fluttering action but this enforces the more waterfront curvature to maintain a minimum of 2.5 degrees of wrap/roll to ensure traction to drive the roll. By adding more rollers, the distance between adjacent rollers is reduced and so is fluttering. Even in the embodiment where the print zone is located at the surface of a roller, fluttering of the web before and after the print zone can also negatively affect print quality. This has been measured up to 44 um of deflection at +/−7 mm at the first and last rows of jets in the process direction.
Consequently, what is desired is a web transport system which reduces undesirable movement or fluttering and troughing of the web, in particular when induced by transport through a print zone. By reducing or eliminating the amount of flutter, print quality of text and images is improved.